/Engine/Source/Developer/TargetPlatform/Public/Common/TargetPlatformBase.h
The implementation will assign an ordinal to each target platform at startup, assigning a value of 0, 1, 2, etc in order to make the ordinals usable as array / bit mask indices.
int32 A unique integer which may be used to identify target platform during the current session only (note: not stable across runs).
virtual int32 GetPlatformOrdinal() const